As a valve opening and closing timing control apparatus having the configuration described above, JP 2014-47778A (Reference 1) discloses a technology of having a torsion spring (a coil spring in Reference 1) over the driven side rotator (an inner rotor in Reference 1) and the driving side rotator (a housing in Reference 1). A biasing direction of the torsion spring is set so as to bias a relative rotation phase of the driven side rotator relative to the driving side rotator in an advance direction.
JP 2013-185459A (Reference 2) discloses a technology of providing a torsion spring (a coil spring in Reference 2) between a driving side rotator (a front plate in Reference 2) and a driven side rotator (a vane rotor in Reference 2) of a valve opening and closing timing control apparatus, thereby biasing the driven side rotator relative to the driving side rotator in an advance direction.
In a concrete configuration of Reference 2, a spring hook is provided on the front plate, a cylindrical bush is fixed to the vane rotor such that a portion thereof is embedded in the front plate, the entire torsion spring is disposed along the inner periphery of the bush, and the torsion spring has one end locked by the bush and the other end locked by the spring hook.
As a configuration in which the biasing force of the torsion spring is transmitted to the valve opening and closing timing control apparatus, for example, as described in Reference 2, when the spring hook is provided on the front plate, a process for attaching the spring hook is required, and by attaching the spring hook, a separate part (the spring hook) is required. In particular, when a configuration in which the spring hook is press-fitted into the front plate of the valve opening and closing timing control apparatus is adopted, an increase in the strength of the front plate, such as an increase in the thickness of the front plate, etc., is required.
In addition, for example, in the spring hook described in Reference 2, a portion of the torsion spring that is close to the front plate is deformed by the action of an external force so as to be lifted from the front plate (to be wholly compressed), the end of the torsion spring may be separated from the spring hook.
In order to cope with this problem, the spring hook may be formed in a long dimension in the direction along the rotation axis of the valve opening and closing timing control apparatus. However, when the spring hook is formed in a long dimension, because the spring hook is not only increased in size, but also is increased in weight, it is conceivable that rotation balance is deteriorated.
Such a problem exists in a valve opening and closing timing control apparatus having a torsion spring without limited to the configurations of Reference 1 and Reference 2, and there is room for improvement.
Thus, a need exists for a valve opening and closing timing control apparatus which is not susceptible to the drawback mentioned above.